Having excellent transparency, heat resistance and mechanical strength, a polycarbonate resin is widely used in the fields of electricity, machines, automobiles and medical use. On the other hand, a polycarbonate resin is widely used for various transparent members owing to the above excellent properties. Above all, attempts are made to apply it to transparent members for automobiles that seek to decrease the weight thereof. These transparent members for automobiles include a headlamp lens, a resin window glass, a rear lamp lens and a meter cover. These members have characteristic features that they have complicated forms and large sizes and that qualities that are demanded for molded articles are very high.
When these articles are produced from a polycarbonate resin composition by an injection molding method, there is a problem that it is difficult to obtain a molded article that has excellent releasability from a mold and a decreased internal strain therein and that is free of breaking. Specifically, when excellent releasability from a mold is provided, an internal strain in a molded article or the breaking of a molded article is liable to take place. On the other hand, when releasability from a mold is decreased, the breaking of a molded article by an increase in resistance against the releasing thereof from a mold is increased. In addition to these problems, it is required to have excellent transparency, resistance to boiling water, resistance to molding heat and weatherability.
The above internal strain in a molded article generally refers to a well known strain that is observed as a streaks-like color change or non-uniformity of denseness and non-denseness through a polarizing plate and that is decreased to some extent by annealing treatment (this strain will be sometimes referred to as “strain-1” for the convenience). On the other hand, there is also a strain that is observed as a non-uniform shadow portion in observation through polarizing plates (this strain will be sometimes referred to as “strain-2” for the convenience) and that is not decreased by normal annealing treatment but sometimes becomes conspicuous in reverse.
The substance of the strain-1 is a strain that individual polymer chains have due to thermal stress. The strain-2 is liable to take place when the molding temperature during molding is high and when a molding machine is large and has a large residence portion. The strain-1 constitutes a factor that causes a cracking in treatment for hard coating or long-term properties, and its decreasing is hence required. The strain-2 has failures such as the occurrence of silver, discoloration and internal cloudiness (white fogging) which may appear on/in a molded article due to a slight increase in a residence time period, and it is required to decrease these.
These transparent members for a vehicle are surface-treated with hard coating treatment, etc., in many cases. In the above hard coating treatment, however, molded articles sometimes get broken. Such a breaking of a molded article also takes place even if the molded article has been subjected to annealing treatment before the hard coating treatment. It is hence considered that some property of a polycarbonate resin composition may promote the breaking of a molded article both during molding and during annealing. It is required to decrease the factor for such a breaking in order to improve a molded article in durability against breaking (to be sometimes simply referred to as “breaking durability” hereinafter).
Conventionally, a method of incorporating a fatty acid ester is widely known as a method of improving the releasability of a polycarbonate resin from a mold, and in particular, glycerin monostearate is often used. In a polycarbonate resin composition into which glycerin monostearate is incorporated, the above strain-2 is clearly observed, and it cannot be said that the polycarbonate resin composition has been sufficient for the durability against breaking.
As a mold release agent for use with a polycarbonate resin, there is also widely used a full ester of a polyhydric alcohol and an aliphatic carboxylic acid, such as pentaerythritol tetrastearate. Concerning a polycarbonate resin composition containing the above full ester, various proposals have been made as well for further improving its product qualities. An ester of a polyhydric alcohol and an aliphatic carboxylic acid will be sometimes simply referred to as “fatty acid ester” hereinafter.
Patent Document 1 discloses a polycarbonate resin composition containing an ester of pentaerythritol in which the OH group content and acid value of an ester compound are made extremely small. However, the primary object of the invention described in the patent document is to decrease the ductility-brittleness transition temperature of the above resin composition (that is, the ductility temperature region is increased).
Further, Patent Document 2 discloses a polycarbonate resin composition comprising a polycarbonate resin and an internal mold release agent having a full ester percentage of 90% or more, an acid value of 0.6 to 1.6, an iodine value of 0.1 to 1.3 and a metal element, Sn, content of 5 to 300 ppm. The above official gazette discloses that when the acid value exceeds 1.6, the resistance to molding heat is decreased, while it does not disclose the sufficient technical findings of a polycarbonate resin composition that satisfies any one of excellent releasability from a mold, a decreased internal strain in a molded article (in particular, strain-2) and improved breaking durability.
As described above, the transparent members for a vehicle require a polycarbonate resin composition having excellent releasability from a mold, a decreased internal strain in a molded article and improved breaking durability in addition to excellent transparency, weatherability and resistance to molding heat. Further, the transparent members for a vehicle are used outdoors in many cases, and in particular exposed to a high-temperature high-humidity state for a long time in the summer season, so that they are required to have resistance to heat and moisture at high temperatures.
Patent Document 3 discloses that a polycarbonate resin composition having excellent transparency, weatherability, resistance to molding heat and releasability from a mold, having a decreased internal strain in a molded article and having durability against breaking can be obtained by adding a specific mold release agent and ultraviolet absorbent to a polycarbonate resin. However, when a phosphorus stabilizer is used for improving resistance to molding heat, there is a problem that the phosphorus stabilizer is converted to an acidic compound due to hydrolysis to deteriorate the resistance of a molded article to heat and moisture.
Patent Document 4 discloses that a polycarbonate resin composition excellent in thermal stability and water resistance can be obtained by adding a combination of a specific phosphorus compound, a phenolic compound and an epoxy compound to a polycarbonate resin. However, there is not any composition that satisfies all of the properties that are required of transparent members for a vehicle.
Meanwhile, a polycarbonate resin composition has a problem that it promotes the corrosion of a mold after its molding is finished. Normally, when its molding is continuously carried out, no corrosion problem is caused since the amount of water (moisture) is small. However, when molding is stopped for a while, the problem of corrosion of a mold takes place. That is considered to be because an acidic substance generated by decomposition of a trace amount of a chlorine-based organic solvent or additive contained in a polycarbonate and an acidic impurity contained in an additive are exposed to moisture in an adhering state thereof to a mold surface, thereby to extremely promoting the corrosion of a mold.
Patent Document 5 discloses that a polycarbonate resin composition having an effect on the inhibition of corrosion of a mold can be obtained by adding a partial ester of a higher fatty acid such as glycerin monostearate and a polyhydric alcohol to a polycarbonate resin.
Patent Document 6 discloses a polycarbonate resin in which the corrosion of a mold is inhibited by adding an alicyclic epoxy compound to a polycarbonate resin. However, there is not any composition that satisfies properties required of transparent members for a vehicle.    (Patent Document 1) JP 02-069556A    (Patent Document 2) JP 2001-192543A    (Patent Document 3) Japanese Patent No. 4046159    (Patent Document 4) Japanese Patent No. 3285690    (Patent Document 5) JP 62-034791B    (Patent Document 6) Japanese Patent No. 2579653